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Science Highlights

The power of tracers: a 3D animation example of Thorium-230 along GEOTRACES North Atlantic Sections

The central component of GEOTRACES is a series of cruises spanning the global ocean. These cruises collect water and analysis of many trace elements and isotopes. The power of the GEOTRACES field work is demonstrated below by an example of 3D animation resulting from data collected on GEOTRACES North Atlantic cruises:

Figure: Distribution of dissolved 230Th along GEOTRACES North Atlantic GA02 and GA03 sections. Click here to view the video in full screen.

This animation illustrates the distribution of dissolved Thorium-230 (230Th) in the North Atlantic Ocean along GEOTRACES sections GA02 and GA03 (Click here to access the GEOTRACES Atlantic Ocean section map). Dissolved 230Th is supplied uniformly throughout the ocean by radioactive decay of dissolved uranium. It is removed by adsorption to particles as they sink to the sea floor. Consequently, the distribution of dissolved 230Th informs us about the location and intensity of the processes that remove many trace elements and other insoluble chemicals from the ocean. In addition, the concentration of dissolved 230Th tends to increase with the age of the water, that is, with time since the water mass was last at the ocean surface. This effect is evident here in that dissolved 230Th concentrations tend to increase from the surface to the bottom, from north to south and from west to east, corresponding to increasing water mass age in the North Atlantic Ocean. Departures from this general trend indicate the locations of unusually intense removal by adsorption to particles. Three such examples can be identified: 1) within the plume of hydrothermal particles surrounding the crest of the Mid Atlantic Ridge, 2) near the sea floor where particles resuspended from the bottom enhance the removal process, and 3) approaching the margins of North America and North Africa where the effects of continental erosion and elevated rates of biological productivity cause high particle abundance. Because processes that regulate the removal of 230Th from the ocean also affect other insoluble chemicals, this illustration of the distribution of dissolved 230Th informs us about the fate of many chemicals in the ocean.

 

Latest recommendations for successful analysis of dissolved osmium in seawater

Analysis of osmium in seawater presents complex challenges, linked to its very low (femtomolar) concentrations and multiplicity of possible oxidation states. Early insights were provided by Karl Turekian's group at Yale where it was realized that osmium tends to concentrate both in oxidizing Fe-Mn nodules and in reducing organic-rich marine sediments. Efforts to directly measure the seawater osmium isotope composition and concentration began in earnest following the developments in early 1990s of highly sensitive N-TIMS and ICP-MS. Initial techniques that attempted to pre-concentrate osmium using column chromatography (Minoru Koide and collaborators at Scripps Institution of Oceanography) and co-precipitation (Mukul Sharma and collaborators at Caltech) were only partially successful due a lack of equilibrium between seawater and tracer osmium. A breakthrough came in 1998, when Sylvain Levasseur in Claude Allegre's group in Paris simultaneously oxidized and pre-concentrated osmium in liquid bromine at 90°C. Oliver Woodhouse and coworkers at the Woods Hole Oceanographic Institution developed another procedure of directly distilling osmium from seawater and sparging it into an ICP-MS. These procedures appeared robust but yielded conflicting results. Subsequent work at Dartmouth (Sharma and collaborators) and Nancy (Maxence Paul and collaborators) has demonstrated that much higher temperatures and longer durations are required to fully equilibrate sample and tracer osmium. The complexities involved in storage of seawater osmium have also become apparent (see link to Eos report below). These findings resulted from U.S. National Science Foundation funded GEOTRACES intercalibration efforts in the Pacific and Atlantic oceans. The new insights call into question much of the earlier data on the marine distribution of this important biogeochemical tracer and raise new issues: How actively is osmium cycled in the water column? What is the relative importance of the various sources? How important are anthropogenic inputs? The workshop on  "Dissolved Osmium Isotope Analysis" held at the Palais de Congrès de Montreal on 24 June 2012 before the annual Goldschmidt Conference summarized the latest recommendations for successful seawater osmium analyses.

Read more: Latest recommendations for successful analysis of dissolved osmium in seawater

Substantial intra-basin variation of the dissolved metal/phosphorus ratio in the different water masses of the Indian Ocean

The first simultaneous, full-depth, and basin-scale section-distribution of dissolved (D) aluminum (Al), manganese (Mn), iron (Fe), cobalt (Co), nickel (Ni), copper (Cu),  zinc (Zn), cadmium (Cd), and lead (Pb) is reported in the Indian Ocean. In addition to widespread co-limitation for phytoplankton production by dissolved iron (DFe) and occurrence of redox-related processes, the authors observe an important variability of the dissolved metal/phosphorus ratio among the water masses within the Indian ocean (up to a factor of 300 between Arabian Surface waters and Lower Circumpolar Deep Water). The Cu/P, Zn/P, and Cd/P ratios are within the same order of magnitude for both phytoplankton and deep water, whereas the Mn/P, Fe/P, and Co/P ratios of phytoplankton can increase 100-fold or more compared to those in deep water. Such results are questioning the validity of using an "extended Redfield ratio" to trace metals. The consistent mechanism yielding these variations remains to be understood...

Meridional section distribution (~70°E) of the DMn/DAl ratio Click on the image to view it larger

Figure: Meridional section distribution (~70°E) of the DMn/DAl ratio

Read more: Substantial intra-basin variation of the dissolved metal/phosphorus ratio in the different water...

A global compilation of dissolved iron measurements: focus on distributions and processes in the Southern Ocean

A data synthesis effort recently compiled over 13,000 observations of dissolved iron concentrations that more than doubled the previous data compilation. A systematic analysis of the distribution of data in the Southern Ocean was performed using four regions, six basins and five depth intervals as a framework. Substantial variability in the depth dependent trends were found between different basins and regions, which were indicative of the possible underlying influence of ocean physics, chemistry and biology. Alessandro Tagliabue's and co-authors (Tagliabue, et al. 2012) analysis was able to highlight where observations are lacking in a particular region or time of year, which they hope will assist future sampling efforts. Overall, more observations have been collected in the past 5 years under the auspices of the International Polar Year and GEOTRACES efforts than were collected in the prior ~15 years. Nevertheless, despite this progress the seasonal cycle of iron that can be extracted from the well-sampled region south of Tasmania remains enigmatic. From over 160 observations, they find little evidence of 'winter recharge' in iron concentrations and instead find the highest iron concentrations to be coincident with the highest phytoplankton biomass levels. This might reflect gaps in seasonal sampling between July and November or the influence of the so-called 'ferrous wheel' in driving the recycling of iron. This clearly highlights the need for more measurements of iron at 'seasonal transitions', even in well-sampled areas.

Read more: A global compilation of dissolved iron measurements: focus on distributions and processes in the...

Neodymium isotopic compositions and rare earth element data evidence boundary exchange in the southwestern tropical and equatorial Pacific

New rare earth element (REE) concentrations and neodymium (Nd) isotopic compositions measured between the southwestern Tropical and the central equatorial Pacific (112 samples) support the hypothesis that the main process driving the geochemical supply in this area is boundary exchange. Nd signatures confirm the major role of the Papua New Guinea coast but also reveal new candidates such as French Polynesia, Vanuatu, Fiji or Galapagos to explain the progressive geochemical enrichment of the waters along their pathways. The positive europium anomaly characterizing the REE patterns of the water masses also trace these continental inputs.

Grenier 2013

Click on the image to view it larger.

Figure: εNd of the dissolved fraction of the upper (top) and lower (bottom) thermocline water masses flowing in the Southwestern and Equatorial Pacific, identified by red dots on the adjacent θ-S plots. Each dot corresponds to a Nd sample and its color refers to its εNd value. Currents are represented by grey arrows. Large red arrows illustrate areas along the water mass pathways where radiogenic supplies likely occur to explain observed εNd changes. The bathymetry shallower than 250m is colored in brown.

Read more: Neodymium isotopic compositions and rare earth element data evidence boundary exchange in the...

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